1. Field of the Invention
The present invention relates to a servo controller for synchronously controlling a master side drive source and a slave side drive source when the servo controller is applied to a machine in which the same machining is repeatedly conducted and, for example, thread cutting or tapping is repeatedly conducted.
2. Description of the Related Art
In general, in the case of machining a male screw on an outer circumferential face of a work, while the work chucked by a primary spindle is being rotated, a predetermined depth of cut is given to a thread cutting tool and the thread cutting tool is linearly moved in the axial direction of the work so as to conduct thread cutting on the work. In this case, in order to prevent the cutting tool from being given an excessively strong cutting force, the depth of cut is given to the cutting tool being divided by several times, that is, cutting of a predetermined depth is repeated by a predetermined number of times so that a perfect thread shape can be formed. In the case of machining a female screw on a work with a tap, the work is fixed onto a table which is moved in X-Y directions, and while the tap attached to the primary spindle is being rotated, the tap is fed in the axial direction of the rotary axis. On the contrary, while the tap is being rotated, the work is fed in the axial direction of the rotary axis. In this way, thread cutting is conducted.
A feed speed of the cutting tool in the case of machining a male screw and a feed speed of the tap in the case of machining a female screw are determined according to the rotary speeds of the work and tap so that the screw can be continuously formed at a predetermined pitch. That is, a movement command (feed speed) of the cutting tool or the tap, which is linearly moved, and a rotation command (rotary speed) of the rotating work or the rotating tap are maintained at a predetermined ratio. Therefore, in this type of thread cutting or tapping, in order for both drive sources (servo motors) to be synchronously driven at a predetermined ratio, the rotation command and the movement command are given by a numerical controller of a machine tool.
An example is explained below in which a screw of 1 mm pitch is machined at 6000 min−1. In this case, a position detecting unit of a feed axis driven by one drive source is 10000 pulse/mm and a position detecting unit of a rotary axis driven by the other drive source is 4096 pulse/rev. When consideration is given to a movement command of the feed axis, the rotary axis is rotated one revolution in 10 ms and the feed axis advances 1 mm per revolution. Therefore, the number of pulses becomes 10000 pulse/10 ms. A feed speed of the feed axis becomes 6 m/min. On the contrary, the number of pulses of the rotary axis becomes 4096/10 ms. Accordingly, a ratio of one drive axis to the other drive axis is K=4096/10000. Accordingly, when a movement command for the drive source to drive the feed axis is multiplied by 4096/10000 and the thus obtained command is made to be a movement command for the drive source to drive the rotary axis, it is possible to machine a screw of 1 mm pitch.
This is an example in which a pair of driving axes are a rotary axis and a feed axis. However, both of the pair of driving axes may be respectively a feed axis. Alternatively, both of the pair of driving axes may be respectively a rotary axis. That is, the present invention is not limited to the above specific embodiment of the drive axis.
In this connection, the official gazette of JP-A-2004-280772 discloses an example of the conventional servo controller in which the respective drive sources of the rotary and the feed axis are synchronously controlled so that a work can be repeatedly machined into the same shape.
As described before, in the case where a male screw is machined on an outer circumferential face of a work or a female screw is machined at a predetermined position of a work with a tap, it is necessary that drive sources for respectively driving a rotary axis and a feed axis are synchronized with each other. It is conventional to conduct machining in such a manner that a movement command of one drive source is multiplied by a specific ratio and the other drive source is driven according to thus multiplied movement command.
However, in the case of machining a male screw or a female screw, as long as the servo characteristics of the drive sources, which are driven synchronously with each other, are the same, the rotary axis and the feed axis have the same positional deviation. Therefore, from the theoretical view point, no synchronization error is caused. However, in the case where inertia of the rotary axis is increased according to an enhancement of the rigidity of the rotary axis or in the case where the rotary axis is rotated at a high speed, the servo characteristic of the rotary axis is inferior to that of the feed axis. Accordingly, a large positional deviation is generated at the time of acceleration and deceleration of the rotary axis. Further, the synchronization error is increased by a cutting disturbance such as a change in the depth of cut of a cutting tool or a shock caused at the time of starting cutting. Furthermore, the synchronization error is increased by the friction. Being affected by the cutting disturbance and friction, the accuracy of machining a screw is deteriorated.
When the positional deviation of each axis can be made to come close to 0 by conducting learning control on the individual positional deviation of both axes, the synchronization error can be made come close to 0. However, although the inertia is high especially on the rotary axis side, it is necessary to rotate the rotary axis at a high speed. For the above reason, there is a high possibility that the learning control cannot exhibit the effect because it is restricted by the maximum torque of a motor. Explanations are made by referring to an example shown by the paragraph number 0004 as follows. When the rotary speed of the primary spindle is 6000−1, the speed of the feed axis is 6 m/min−1, and the rotation·linearity conversion coefficient (for example, the pitch of a ball screw) is 10 mm/rev. In this case, the rotary speed of the feed axis is 600 min−1 which is 1/10 of the rotary speed of the rotary axis. In this way, even when the axes are driven by the same synchronization command, the speed of the rotary axis must be higher than that of the feed axis by 10 times. Therefore, even when both axes are rotated by the same acceleration and deceleration time constant, a higher torque must be given to the rotary axis, which is very difficult from the practical viewpoint. Therefore, it is reasonable that the synchronous error is learned and controlled by the feed axis because this method is more advantageous from the viewpoints of speed and inertia than the method of learning the positional deviation of both axes.